The liquid-in-stem thermometer in its simplest form comprises of a transparent tubular stem with a capillary bore at one end, a bulb fused to the stem at the other end, and a thermometric liquid in the bulb. Typically, a graduated scale is engraved in or attached to a portion of the stem. Sometimes, the stem is encased in an Envelope of transparent material and the graduations are marked on a scale which is located inside the envelope. As is well known, the expansion or contraction of the liquid in the bulb caused by changes in the temperature of the liquid forces the liquid to traverse the capillary of the stem to or from the bulb. Provided that the thermometer has been properly calibrated, the position of the meniscus of the liquid column in the capillary on the graduated scale indicates the temperature being measured.
When thermometers are designed for use in clinical applications the capillary in the stem has a constriction near the bulb. When the bulb is brought into thermal contact with a warm body, a column of the thermometric liquid freely rises in the capillary of the stem and when the bulb is moved away from the source of heat, the thermometric liquid in the bulb contracts but the liquid column in the capillary above the constriction is not withdrawn into the bulb, thus providing the maximum registration action. Such thermometers are reset by centrifugation or manual shake down.
There are also other ways of constructing clinical thermometers with maximum registration action, such as through the use of electronic, electrical, mechanical, optical and tympanic thermometers. However, liquid-in-stem clinical thermometers are very popular because they are inexpensive, accurate, fairly trouble-free and give reproducible results.
Mercury is the most commonly used thermometric liquid and glass is a commonly used material of construction of the maximum registration thermometer. However, such thermometers have several drawbacks. Being made of glass, potential breakage is a problem. Secondly, breakage results in spilled mercury and mercury is known to be toxic. Several methods in the prior art have been directed towards i) preventing spillage and ii) replacing mercury with a non-toxic, environment-friendly thermometric liquid. Several thermometric liquids have been proposed as alternatives to mercury. These include metallic alloys, alcohol and glycerol. However these thermometric liquids still have one or more drawbacks in terms of toxicity, poor accuracy, split in the liquid column, loss of maximum registration, etc.